The format specifiers used in `String.init(format:)` are derived from `[NSString format:]` (or C's *
`printf`) and described in this doc.
Generally, `ll` stands for `long long`, which represents 64-bit integer in Apple's platforms. (Both in 32-bit and 64-bit.)
And in Swift, `long long` is equivalent to `Int64` (or `CLongLong`).
So, use `%lld` for `Int64`.
And `%d` takes C-`int`, which is equivalent to `Int32` (or `CInt`) in Swift. (Always represents 32-bit, both in 32-bit and 64-bit platforms.)
`Swift.Int` is equivalent to C-`long` (or `long int` or `NSInteger`), which is 32-bit in 32-bit platforms and 64-bit in 64-bit platforms.
And the right format for C-`long` is `%ld`.
If you write your code only for 64-bit platforms, `%lld` and `%ld` are equivalent, but I recommend you to use `%ld` for `Swift.Int`.
Thanks a lot for the detailed reply